Sigma is population std. deviation or std. deviation of sampling distribution?

Hello. I had a confusion about formula of confidence interval. What is sigma in the formula, is it the population std. deviation or the std. deviation of sampling distribution? If it is std. deviation of sampling distribution why did teacher say population std. deviation is needed?

At first glance, I believe it’s the sampling distribution.
Please give a video title and time mark where the video says otherwise.

In week 4 Lesson 1 video named unknown std. deviation. Instructor in the begining only says “we’ve assumed that we know the standard deviation of the total population.” While in video of Calculation steps he says sigma is Std. error which is std. deviation of sampling distribution.

In the first minute of that video, the instructor says it’s the population standard deviation. But I think that’s incorrect, it would be consistent with the other lectures if he referred to the sample distribution.